The physical manipulation of single cells selected on the basis of their individual properties plays an important role in experimental cell biology and immunology, as well as in other areas of biomedical research. Two techniques, flow sorting and micromanipulation, have been used in a wide, albeit mutually exclusive, way for the purposes of cell separation or positioning. Flow sorting allows fast, automated, and essentially nonmechanical separation of cells with given optical and electrical phenotypes. However, both electrostatic and fluidic switching flow sorters divert a volume of fluid containing the cell of interest, rather than the cell itself, and thus, have limited positional accuracy. Micromanipulators can position selected cells with micron accuracy but employ mechanical devices requiring relative large open volumes for unhindered operation.
Optical trapping is a known technique for levitating, positioning and transporting microscopic particles by means of the pressure exerted by one or several laser beams. Particles that can be manipulated by optical trapping include biological cells and cell constituents, such as chromosomes. Optical trapping is particularly important in applications such as the creation of hybrid cell lines, the separation of rare cells, such as fetal cells in maternal blood, and the separation of chromosomes and chromosome fragments.
Optical trapping is based on the transfer of momentum between microscopic particles and the photons they scatter. While the forces produced by this interaction are extremely small, so are the other forces, such as weight and viscous drag, which act on a microscopic particle suspended in a stationary or slowly moving fluid. Optical trap devices are disclosed in U.S. Patents, such as U.S. Pat. Nos. 4,887,721 and 5,100,627.
Under suitable conditions, the interaction between a collimated laser beam and a microscopic particle results in a radial force proportional to the gradient of beam intensity and in the direction of the intensity gradient, and in an axial force proportional to beam intensity and directed along the beam axis. The radial force can act as a restoring force that traps the microscopic particle on the beam axis. In a strongly focused laser beam, as taught in the prior art, the axial force may change sign at a point close to the beam waist, and a full three-dimensional trap results. Short focal length optics are provided and the trap is limited to a small volume close to the focus point. Thus, the particles are trapped about the focus and cannot move along the beam axis for transport. See U.S. Pat. No. 4,887,721, issued Dec. 19, 1989 and U.S. Pat. No. 5,100,627, incorporated herein by reference.